Reduction of NOx is of increasing concern as emission guidelines become more stringent. Types of emission treatments may include nitrogen oxide storage catalysts, also known as NSR catalysts, (e.g., NOx storage and reduction catalyst) or a lean NOx trap (LNT) may represent two exemplary aftertreatment devices for the catalytic conversion of nitrogen oxides. An alternative technology may include a selective catalytic reduction (SCR) device, which may utilize a reductant solution applied thereon to reduce nitrogen oxides.
LNT catalysts may store nitrogen oxides at temperatures below a threshold, wherein the nitrogen oxides stored thereon may be reduced in the presence of a rich air/fuel mixture. The hydrocarbons and carbon monoxide may function as reducing agents. Contrastingly, the SCR device may increase reduction efficiency at temperatures above the threshold, resulting in the SCR and LNT working in tandem to treat nitrogen oxide emission at a greater range of exhaust gas temperatures.
Thus, to meet the more stringent emission guidelines, it may be desired to pair the LNT catalyst and the SCR device in an exhaust gas treatment arrangement to increase a temperature range in which the nitrogen oxides may be reduced. However, this may lead to problems. In one example, the rich mixture used to reduce nitrogen oxides captured by the LNT catalyst may leak through the LNT, wherein the rich exhaust gas mixture may reach the SCR device, which may decrease a longevity of the SCR device.
Other attempts to address rich ageing of the SCR catalyst include arranging an air supply device upstream of the SCR. One example approach is shown by Gandhi et al. in U.S. Pat. No. 8,776,498. Therein, an air injection system and an oxygen storage capacity (OSC) device are arranged between a LNT and an SCR catalyst, wherein the air injection system and OSC work in tandem to decrease exposure of the SCR catalyst to rich exhaust gas.
However, the inventors herein have recognized potential issues with such systems. As one example, the air injection system may increase packaging constraints and may decrease fuel economy. Furthermore, OSC have a limited capacity, wherein once the OSC is depleted of oxygen the exhaust gas mixture upstream of the SCR catalyst may return to a rich mixture, thereby degrading the SCR device.
In one example, the issues described above may be addressed by a system comprising an exhaust passage comprising a lean-NOx trap (LNT) arranged upstream of a selective-catalytic-reduction (SCR) catalyst with respect to a direction of exhaust gas flow, an oxygen storage component arranged between the LNT and the SCR, and a controller with computer-readable instructions stored thereon that when executed enable the controller to limit a rich operation of an engine in response to an exhaust temperature and an oxygen load of the oxygen storage component. In this way, the rich ageing of the SCR in the presence of hot exhaust gas may be preempted.
As one example, the rich operation is limited in response to the exhaust gas temperature being higher than a limit temperature. If the exhaust gas temperature is less than the limit temperature, then the exhaust gas may not be hot enough to degrade the SCR, even if the exhaust gas is rich. As such, a rich operation which further includes an exhaust gas temperature less than the limit temperature may not be limited based on the oxygen load of the oxygen storage component. Additionally, feedback from one or more sensors arranged between the LNT and the SCR may be ignored, which may increase fuel economy.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.